Drive arrangements for mining machines

ABSTRACT

A mining machine travels along a haulage chain by using a series of toothed sprockets in a drive of the machine to engage the chain. The drive is reversible and the drive arrangement causes the currently leading sprocket to drive the machine irrespective of the direction of travel. This reduces the risk of damage to the chain.

United States Patent 1191 1111 3,892,184 Kenn et a1. July 11, 1975 [54] DRIVE ARRANGEMENTS FOR MINING 2,841,093 7/1958 Camp 104/235 2,918,020 12/1959 Henderson et a1 104/172 C MACHINES 3,012,520 12/1961 Curtis 104/178 X [75] Inventors: Robert Charl s Kenn, Lichfield; 3,188,977 6/1965 Viktorsson 104/178 x David Arthur Tate, Burton Upon 3,474,739 10/1969 Van Brakel 104/172 B Trent, both of England 3,572,563 3/1971 Oliver 104/172 B X 731 Assignee: c031 Industry (Patents) Limited, F REIGN PATENTS OR APPLICATIONS London, England 1,091,886 11/1967 United Kingdom 104/178 Filed g 8 1973 335,036 9/1930 United Kingdom 104/178 [21] Appl. No.: 386,648 Primary Examiner1\/l. Henson Wood, Jr.

Assistant Examiner-Andres Kashnikow 52 11.5. C1. 104/147 R; 74/245 R; 74/245 0; Agent Flrm james Wray 104/172 B; 104/178; 104/235 511 In1.Cl B611) 13/00 [57] ABSTRACT [58] Field of Search 104/172 R, 172 C, 172 B, A mi ing m hin tr v ls long a haulage chain by 104/165, 178, 147 R, 236, 237, 172 ET, 172 using a series of toothed sprockets in a drive of the S 235; 74/245 R 245 C 217 C 218 220 machine to engage the chain. The drive is reversible 226, 243 R, 243 C, 243 H and the drive arrangement causes the currently leading sprocket to drive the machine irrespective of the [56] References Cit d direction of travel. This reduces the risk of damage to UNITED STATES PATENTS the Cham- 2,642,005 6/1953 Cooper 104/172 B x 1111 Claims, 8 Di'awiilg Figures DRIVE ARRANGEMENTS FOR MINING MACHINES This invention relates to drive arrangements for mining machines and in particular to drive arrangements comprising sprockets which in operation engage haulage chains extending along the paths of the machines.

With a known mining machine, the drive arrangement comprises a drive sprocket and at least one idler sprocket around which the haulage chain passes in sequence.

In operation substantially all wear and damage to the links of the chain occurs when the chain engages or disengages a sprocket, the amount of wear or damage increasing rapidly as the chain tension increases. Thus, it follows that if relatively highly tensioned chain is passed around a sprocket the wear and damage caused to the chain is greater than if relatively low tensioned chain is passed around the sprocket.

Unfortunately, with the known drive arrangement it is necessary when the machine is travelling in one direction that the drive sprocket pulls on the section of chain passing around the idler sprocket. Thus this section of chain has a relatively high tension causing the chain links to wear quickly and become damaged and the operational life of the chain to be reduced. This is especially so when relatively highly tensioned chain is wound in a clockwise direction around one of the sprockets and in a counter clockwise direction around the other sprocket. Further, each chain link which is made continuous by welding is subjected to reverse bending which at relatively high tension tends to cause failure at the weld due to fatigue.

An object of the present invention is to provide a drive arrangement for a mining machine which reduces the tendency of the chain to become worn or damaged.

According to the present invention a drive arrangement for a mining machine adapted to haul itself to and fro along a haulage chain extending along the machines path comprises a plurality of sprockets arranged for engagement with the chain, and means adapted to drive at least the currently leading sprocket irrespective of the direction of travel of the machine along the chain.

Preferably, the drive means enables the drive of at least two of the sprockets to be driven simultaneously.

The drive means may be such that the currently leading sprocket exerts substantially all the driving force required to haul the machine along the chain while the currently trailing sprocket exerts only sufficient force to haul the relatively slack chain around the trailing sprocket.

Alternatively, the drive means may selectively control said drives so that only the currently leading sprocket is driven.

Preferably, two of the sprockets are mounted for rotation about axes arranged normal to one another so that the two sprockets drivably engage alternate links of the chain, respectively.

Preferably, each of said drives comprises a hydraulic motor.

Advantageously, both the hydraulic motors are fed with pressure fluid from a single pump.

Gearing may be provided between each motor and its associated drive sprocket and the gearing may include an epicyclic gear.

By way of example only, seven embodiments of the invention will be described with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic plan of a part of a first embodiment of a drive arrangement for a mining machine, constructed in accordance with the present invention;

FIG. 2 is a diagrammatic plan of a part of a second embodiment of a drive arrangement for a mining machine, also constructed in accordance with the present invention;

FIG. 3 is an incomplete plan of a third embodiment of drive arrangement for a mining machine, also constructed in accordance with the present invention;

FIG. 4 is a section along the line IV IV of FIG. 3;

FIG. 5 is a hydraulic circuit for the third embodiment of drive arrangement;

FIG. 6 is a diagrammatic view of a part of a fourth embodiment of drive arrangement for a mining ma chine constructed in accordance with the present invention;

FIG. 7 is a hydraulic circuit diagram circuit for a fifth embodiment of drive arrangement; and

FIG. 8 is a hydraulic circuit for a sixth embodiment of drive arrangement.

FIG. 1 shows a part of the haulage drive arrangement of a well known shearer type mining machine for winning coal from the face by repeatedly hauling itself to and fro along a flexible round link chain 2 extending along the path of the machine, the ends of the chain 2 being attached to anchored chain tensioning devices with the chain initially at a relatively low tension e.g. 2 tons. As the machine hauls itself along the face a rotary cutter drum (not shown) mounted on the machine engages the coal face and wins the coal which is loaded onto an armoured face conveyor (not shown) in well known manner.

The haulage drive arrangement comprises a motor 4 (only a part of which is shown) drivably engaging gears within a gearbox 6, and two sprockets 8, and 10 mounted on the gearbox 6. The sprockets 8, 10 are arranged so as to sequentially engage the chain 2.

The sprockets 8, 10 are connected to the gearbox 6 so that when the machine is travelling in one direction, the sprockets 8, 10 i.e. the currently leading sprocket, is selected to be the drive sprocket while the other sprocket l0, 8 is an idler sprocket. When the machine is travelling in the opposite direction the sprocket 8, 10 which was initially the drive sprocket now becomes the idler sprocket and the other sprocket 10, S i.e. the now leading sprocket, is selected as the drive sprocket. A gear control lever 7 is provided on the gearbox 6 to enable an operator to select which of the sprockets 8 or W is the driven. The sprockets 8, 10 are arranged so that in both directions of machine travel the chain links passing around the idler sprocket are at a relatively low tension determined by the initial tension applied to the chain and by the component of the weight of the chain acting at the machine. The tension need be only sufficient to ensure that the chain is pulled around the idler sprocket away from the drive sprocket.

When the machine operator starts the machine for movement in the direction indicated by arrow A, operation of the machine control selects sprocket 8 as the drive sprocket and sprocket 10 as the idler sprocket. The drive sprocket 8 is rotated in a clockwise direction as seen in FIG. 1 so that the section of chain in advance of the machine becomes subjected to a relatively high tension e.g. twelve tons. The tensioned links of the chain are engaged by the teeth of the drive sprocket 8 and wound around the sprocket until they are fed towards the idler sprocket 10. By the time the links of the chain disengage thedrive sprocket 8 they have a relatively low tensionwhich is due to the tensioning devices at the end of the chain and which is maintained throughout the engagement of the links with the idler sprocket 10. Thus the wear and damage caused to the links of the chain as they pass around the idler sprocket I In modified drive arrangements the selection of the drive sprocket is achieved by hydraulic control means which replacethemechanic'al gear control lever. An example of such hydraulic control means is described with reference to the fourth embodiment of drive arrangement (see later in this specification).

FIG. 2 shows a second embodiment of drive arrangement for a shearertype of mining machine which is similar to that described above with reference to FIG.

1, the major difference being that the axis of the sprocket l is turned through 90950 that it is substantially horizontal. With this second embodiment the teeth of the two sprockets 8 and' l0 drivably engage alternate links of the chain, respectivelyie. sprocket 8 drivably engages only the vertical links, the horizontal links forming only tie links and the sprocket l0drivably engages only the horizontal links, the vertical links now forming the tie links. During the passage of the links around a sprocket it is only the drivably engaged links which tend to wear and become damaged, the tie links being relatively unaffectedfTlius' it willbe seen that .with, the axes of the sprockets Sand mounted at right angles with respect to each other,' the wearand damage suffered by the chain links will be spread evenly overall the linksresultingin prolonged operational life of the chain. I g 7 With this second embodiment of drive arrangement I l the links are not subjected to reverse bending as they 'sequentially pass around the sprockets. Thus the operational life of the chairi is further prolonged. f y In modifications of the first and second embodiment controlmeans may be provided which control the drive mechanism so that the currentlyleading sprocket exerts substantially all the driving force required to haul the machine along the chainwhile the currently trailing sprocket exerts only sufficient forces to haul the relatively slack or low tensioned chain around the trailing tension just sufficient to haul the chain around the sprocket and prevent jamming.

FIGS. 3, 4 and 5 of the drawings showa third embodiment of haulage drive arrangement of a well known shearer type mining machine 21 (only a part of which is shown) for winning coal from a longwall face by repeatedly hauling itself to and'fro along a flexible round link chain 22 extending along the path of the machine, the ends of the chain 22 being anchored with the chain initially slack or at a relatively low tension e.g. less than 1 ton. As the machine 21 hauls itself along the face a rotary cutter drum (not shown) mounted on the machine engages the coal face and wins the coal which is loaded onto an armoured face conveyor (not shown) in well known manner.

The haulage drive arrangement comprises two drive sprockets 23, 24 arranged to drivably engage the chain 22 in sequence; which sprocket is the. leading sprocket and which is the trailing sprocket at any one time depends upon the direction of machine travel. The sprockets 23, 24 are provided with teeth 41 which engage the chain 22in well known manner. The sprockets 23, 24 are driven by identical independent drive mechanisms 25, 26 respectively. FIG. 3 shows only partsv of the drive mechanisms 25 and 26, FIG. 4 shows a section through the sprocket 23 and the drive mechanism 25 and FIG. 5 is a hydraulic circuit diagram of the drive arrangement. Each drive mechanism includes an epicyclic gear 27, atrain of spur gears 28 and a hydraulic motor 29. Both the hydraulic motors 29 are fed simultaneously by pressure fluid supplied from a single variable delivery pump 42 (see FIG. 5) housed in the machine body anddriven by the machine's electric motor 4. As can be seen from FIG. 5 the "flow of pressure fluid from the pump 42 is fed to the two motors 29 which are arranged in parallel, the flow being controlled by a control handle 43 capable of being moved from a central Off" position to Forward or Reverse" positions. The amount of movement of the handle away from the Off position determines the quantity of pressure fluid fed to the motors and, therefore, determines the speed of the machine along the chain.

Each epicyclic gear 27 comprises a planet carrier 30 rigid with the associated drive sprocket 23 or 24, three planets 31 arranged to engage an outer fixed annulus 42 and a sun 32 which is fixedly mounted on a shaft 33 with a gear wheel 34 arranged to engage the associated train of spur gears 28 comprising gear wheels 35, 36, 37, 38 and 39. The gear wheel 39 is engaged by a pinion wheel 40 fixedly mounted on the associated motor 29.

In operation, the operator starts the pump 42 which upon actuation of the control handle 43 simultaneously feeds pressure fluid to both the hydraulic motors 29 so that they rotate in opposite directions and cause both the drive sprockets 23, 24 to rotate and drivably engage sprocket. One example of such control means is de- I scribed with reference to the fourth embodiment of trailing sprocket is relatively slack orat a relatively low the chain 22. Since the drive arrangement comprises two drive sprockets 23, 24 the pull exerted by each sprocket for a given machine pull is approximately one half the pull which would be exerted by a single sprocket for the same machine pull. For example, if a machine pull of 20 tons is desired, with a drive arrangement as shown in FIG. 3 the pull exerted by each sprocket 23, 24 would be 10 tons.

As both sprockets 23, 24 are driven it is not necessary for the section of the chain 22 on the'trailing side of the machine 21 to be tensioned in order to rotate the sprocket. With a drive arrangement constructed in accordance with the present invention the chain 22 is not I required to rotate sprocket 23 or 24 and thus the section of the chain 22 on the trailing side of the machine 21 can be slack or at a very low tension. As previously explained this means for a given machine pull the tension in the section of the chain 22 in advance of the machine 21l is kept to a minimum. Thus the operation life of the chain 22 is prolonged and danger and delays arising through chain breakage are reduced.

Another advantage of a drive arrangement constructed in accordance with the present invention is that because the drive mechanism 27 for the sprockets 23 and 24 are independent of each other, the sprockets can momentarily rotate at different speeds so that if the links of the chain 22 fed from the leading drive sprocket 23 or 24 are of varying length, the trailing sprocket 24 or 23 will momentarily rotate at a different speed so that the section of the chain 22 between the two sprockets 23, 24 is kept at a constant tension. This ensures that bunching of the chain 22 between the sprockets 23, 24 is prevented and no resultantjamming of the chain can occur.

Chain guide plates (not shown) may be provided to ensure that the section of the chain 22 on the trailing side of the machine 21 is guided off the sprocket 23 or 24.

In modification, the gearing may be dispensed with and relatively slow speed motors may be directly connected to the drive sprockets.

in further modification, means are provided so that the pull exerted by the trailing sprocket is less than that exerted by the leading sprocket. The pull exerted by the trailing sprocket may be only sufficient to guide the chain away from the leading sprocket. One example of hydraulic means for achieving such a further modification is described with reference to the fourth embodiment described later in the specification.

Referring now to FIG. 6 of the drawing which shows a part of the fourth embodiment of drive arrangement for a mining machine.

The drawing shows a part of the haulage drive arrangement of a well known shearer type mining machine for winning coal from the face by repeatedly hauling itself to and fro along a flexible round link chain 2 extending along the path of the machine, the ends of the chain 2 being anchored with the chain ini tially at a relatively low tension e.g. two tons. As the machine hauls itself along the face a rotary cutter drum (not shown) mounted on the machine engages the coal face and wins the coal which is loaded onto an armoured face conveyor (not shown) in well known manner.

The haulage drive arrangement comprises a motor 4 (only a part of which is shown) drivably engaging gears within a gearbox 6, and three sprockets 8, 9 and are arranged so as to sequentially engage the chain 2, the middle sprocket 9 being an idler sprocket.

The sprockets 8, 10 are driven sprockets connected to the gearbox 6 so that when the machine is travelling in one direction, the currently leading sprocket 8 or 18 is selected to be the drive sprocket while the other sprocket l0, 8 is an idler sprocket. When the machine is travelling in the opposite direction the sprocket 8, it]! which was initially the drive sprocket now becomes the idler sprocket and the other sprocket 1t), 8 is selected as the drive sprocket. The sprockets 8, M) are driven sprockets connected to the gearbox 6 so that when the machine is travelling in one direction, the currently leading sprocket 8 or lit) is selected to be the drive sprocket while the other sprocket lit), 8 is an idler sprocket. When the machine is travelling in the opposite direction the sprocket 8, M) which was initially the drive sprocket now becomes the idler sprocket and the other sprocket lltl, 8 is selected as the drive sprocket. A gear control lever 7 is provided on the gearbox 6 to enable an operator to select which of the sprockets 8 or M) is driven. The sprockets 8, 9 and are arec arranged so that in both directions of machine travel the chain links passing around the idler sprockets are at a relatively low tension determined by the initial tension applied to the chain by the tensioning devices attached to the end of the chain and by the component of the weight of the chain acting at the machine. The tension need be only sufficient to ensure that the chain is pulled around the idler sprocket away from the drive sprocket.

When the machine operator starts the machine for movement in the direction indicated by arrow A, operation of the machine gear control lever 7 selects sprocket 8 as the drive sprocket and sprocket lltl as an idler sprocket along with idler sprocket 9. The drive sprocket 8 is rotated in a clockwise direction as seen in the drawing so that the section of chain in advance of the machine becomes subject to a relatively high tension e.g. twelve tons. The tensioned links of the chain are engaged by the teeth of the drive sprocket 8 and wound around the sprocket until they are fed towards the idler sprockets 9 and 110. By the time the links of the chain disengage the drive sprocket 8 they have a relatively low tension, which tension is maintained throughout the engagement of the links with the idler sprockets 9 and W. Thus the wear and damage caused to the links of the chain as they pass around the idler sprocket tend to be small leading to a prolonged operational chain life.

in order for the machine to travel in the opposite direction to that indicated by arrow A, the operator moves the gear control lever 7 such that the sprocket it) becomes the drive sprocket and is driven in a clockwise direction as seen in the drawings. Again as the machine is hauled along the face, relatively low tensioned chain is fed from the drive sprocket lltl towards the idler sprocket 9 and 8.

ln modifications, mechanical gear control means may be provided which control the drive mechanisms so that the currently leading sprocket 8 or T0 exerts substantially all the driving force required to haul the machine along the chain while the currently trailing sprocket lit) or 8 exerts only sufficient forces to haul the relatively slack or low tensioned chain around the trailing sprockets 9 and llt) or 9 and 8.

When the direction of movement of the machine along the face is reversed the gear control means reverses the operation of the drive mechanisms so that the now leading sprocket exerts substantially all the driving force. Thus in both directions of travel of the machine along the face the chain passing around the currently trailing sprocket is relatively slack or at a relatively low tension just sufficient to haul the chain around the sprocket and prevent jamming.

Referring now to lFlG. 7 which shows a hydraulic circuit of a fifth embodiment of drive arrangement constructed in accordance with the present invention.

The fifth embodiment of drive arrangement is described with reference to a three sprocket arrangement similar to that shown in FIG. 6 although it is to be understood that the drive arrangement is suitable for driving any configuration of sprockets e.g. two sprockets as shown in FIGS. 1 or 2 or configurations where more than one intermediate idler sprocket is provided, e.g. to guide the chain along a preselected path.

The fifth embodiment of drive arrangement drives only the currently leading sprocket.

The two end sprockets (not shown in FIG. 7) are drivably connected to drive shafts 50 and 51 of two fixed displacement hydraulic motors 52 and 53. The

displacement of each of the motors is selected to give the same chain speed on each sprocket for a given flow rate of pressure fluid. As can be seen in FIG. 7 the motors 52 and 53 are hydraulically connected and a variable delivery pump 54 supplies pressure fluid to the two motors arranged in series. A fixed displacement pump 55 is provided to make up leakage through two check valves 56 and 57 and to maintain a desired pressure at the inlet of the pump 54, the inlet pressure being regulated at the desired pressure by a relief valve 58 regardless of the direction of flow of pressure fluid which is controlled by a handle 59 provided on the variable delivery pump 54.

The pumps 54 and 55 are drivably connected to the machines electric motor via gearing 60.

In operation, the handle 59 is moved so that pressure fluid is fed to rotate the desired motor 52 or 53 so that the machine is driven in the desired direction, the driven motor being associated with the currently leading drive sprocket. The pressure fluid supplied from the pump 54 will drive the currently leading motor and associated sprocket and will then exhaust to the currently trailing motor before returning to the pump 54. Leakage and compressibility due to load in the currently tated at a speed faster than the speed associated with the pressure fluid exhausted from the currently leading motor. Thus, the currently trailing motor will not generate any torque or back-pressure.

A second relief valve 61 is provided to by-pass excess pressure fluid past the trailing motor and thus, limit the inter-motor pressure to a low value in those cases where leakage and compressibility affects are relatively small e.g. when the machine is travelling along the face without cutting. The pressure fluid from the relief valve 61 is fed through check valve 56 or 57 to the feed for the pump 54.

When the direction of machine travel is reversed by movement of the handle 59 the flow of pressure fluid will be reversed to drive only the currently leading 1 motor and associated sprocket.

similar to that shown in FIG. 6 although it is to be understood that the drive is suitable for driving any configuration of sprockets e.g. the two sprockets as shown in FIGS. lor 2 or configuration of more than one intermediate idler sprocket.

The sixth embodiment of drive arrangement drives both the currently leading and trailing sprockets and can be adjusted' so that the currently trailing sprocket exerts a preselected thrust. This preselected thrust exerted by the currently trailing sprocket may vary from one half the total driving thrust exerted by the drive arrangement. Alternatively, the thrust exerted by the currently trailing sprocket may be only sufficient to haul the relatively slack chain around the currently trailing and idler sprocket.

The sixth embodiment of drive arrangement is similar to the fifth embodiment of drive arrangement previously described with reference to FIG. 8. However, in the sixth embodiment of drive arrangement a second variable displacement pump 62 is provided. The control handle 59 is arranged to simultaneously control both pumps 54 and 62. The pump 62 is drivably connected by gears 63 to the machines electric motor and is hydraulically connected to the main pipe interconnecting the two motors 52 and 53. A pressure relief valve 64, provided to limit the maximum pressure to a preselected value, is hydraulically connected through check valves 56 and 57 to the flow pipes on either part of the main pump 54 and to the low pressure relief valve 58.

In operation of the sixth embodiment of drive arrangement, the pump 62 makes up for the leakage and compressibility losses in the currently leading motor circuit and provides a desired fluid pressure and flow at the intake of the currently trailing motor which is thereby driven to exert the desired thrust. The proportion of the total driving thrust exerted by the trailing sprocket can be adjusted by adjustment of the setting of the relief valve 64. The distribution of the total driving thrust exerted by the currently leading and trailing sprockets is independent of the direction of rotation for any particular setting of the relief valve 64.

The speed of the machine is controlled by the handle 59 and the delivery of fluid from pump 62 is automatically controlled to zero when the drive is idling so that power wastage and chain tension are minimized while the machine is stationary.

From the above description is will be seen that the present invention provides a drive arrangement for a mineral mining machine which requires only relatively low tensioned chain to be passed around the current trailing sprocket.

We claim:

1. A drive arrangement for a machine adapted to haul itself to and fro along a haulage chain extending along a machine's path, comprising a machine, a plurality of sprockets mounted on the machine and arranged for engagement with the chain, and drive means connected to the sprockets and adapted to drive at least a currently leading sprocket irrespective of direction of travel of the machine along the chain.

2. The drive arrangement of claim 1 in which the sprockets are mounted on respectively normally oriented axes.

3. A drive arrangement for a machine adapted to haul itself to and fro along a haulage chain extending along a machines path, comprising a machine, a plurality of sprockets mounted on the machine and arranged for engagement with the chain, and drive means connected to the sprockets and adapted to drive at least the currently leading and currently trailing sprockets irrespective of direction of travel of the machine along the chain.

4. A drive arrangement as claimed in claim 3, in which the drive means is adapted to drive the currently leading sprocket so that it exerts substantially all the driving force required to haul the machine along the chain while the currently trailing sprocket exerts only sufficient force to haul the relatively slack chain around the trailing sprocket.

5. A drive arrangement as claimed in claim 4, in which adjustment means are provided for adjusting the proportion of the total thrust exerted by each driven sprocket.

6. The drive arrangement of claim 3 wherein the drive means comprise first and second driving means connected to sprockets arranged for remote forward and rearward engagement with the chain.

7. The drive arrangement of claim 6 further comprising first and second power conduit means respectively connected to the first and second driving means, and a reversible power source connected to the power conduit means for supplying power to one conduit means.

8. The drive arrangement of claim 7 further comprising a third conduit interconnecting the driving means whereby exhaust power is supplied to one driving means connected to the currently leading sprocket through the one power conduit means, and exhaust from the one driving means is supplied to the other driving means through the third conduit means.

9. The drive arrangement of claim 8 further comprising relief means connected to the third conduit and to an inlet of the power means for releasing pressure above a predetermined level from the third conduit.

10. The drive arrangement of claim 9, wherein the relief means is adjustable, and further comprising booster power means connected to the third conduit for increasing power in the third conduit to a maximum permitted by adjustment of the relief means.

11. A drive arrangement for a mining maching adapted to haul itself to and fro along a haulage chain extending along a machines path, comprising a machine, a plurality of sprockets mounted on the machine and arranged for engagement with the chain, and drive means connected to the sprockets and adapted to drive only the currently leading sprocket irrespective of direction of travel of the machine along the chain.

l= l= l =l 

1. A drive arrangement for a machine adapted to haul itself to and fro along a haulage chain extending along a machine''s path, comprising a machine, a plurality of sprockets mounted on the machine and arranged For engagement with the chain, and drive means connected to the sprockets and adapted to drive at least a currently leading sprocket irrespective of direction of travel of the machine along the chain.
 2. The drive arrangement of claim 1 in which the sprockets are mounted on respectively normally oriented axes.
 3. A drive arrangement for a machine adapted to haul itself to and fro along a haulage chain extending along a machine''s path, comprising a machine, a plurality of sprockets mounted on the machine and arranged for engagement with the chain, and drive means connected to the sprockets and adapted to drive at least the currently leading and currently trailing sprockets irrespective of direction of travel of the machine along the chain.
 4. A drive arrangement as claimed in claim 3, in which the drive means is adapted to drive the currently leading sprocket so that it exerts substantially all the driving force required to haul the machine along the chain while the currently trailing sprocket exerts only sufficient force to haul the relatively slack chain around the trailing sprocket.
 5. A drive arrangement as claimed in claim 4, in which adjustment means are provided for adjusting the proportion of the total thrust exerted by each driven sprocket.
 6. The drive arrangement of claim 3 wherein the drive means comprise first and second driving means connected to sprockets arranged for remote forward and rearward engagement with the chain.
 7. The drive arrangement of claim 6 further comprising first and second power conduit means respectively connected to the first and second driving means, and a reversible power source connected to the power conduit means for supplying power to one conduit means.
 8. The drive arrangement of claim 7 further comprising a third conduit interconnecting the driving means whereby exhaust power is supplied to one driving means connected to the currently leading sprocket through the one power conduit means, and exhaust from the one driving means is supplied to the other driving means through the third conduit means.
 9. The drive arrangement of claim 8 further comprising relief means connected to the third conduit and to an inlet of the power means for releasing pressure above a predetermined level from the third conduit.
 10. The drive arrangement of claim 9, wherein the relief means is adjustable, and further comprising booster power means connected to the third conduit for increasing power in the third conduit to a maximum permitted by adjustment of the relief means.
 11. A drive arrangement for a mining maching adapted to haul itself to and fro along a haulage chain extending along a machine''s path, comprising a machine, a plurality of sprockets mounted on the machine and arranged for engagement with the chain, and drive means connected to the sprockets and adapted to drive only the currently leading sprocket irrespective of direction of travel of the machine along the chain. 